1. Field of the Invention
This invention relates generally to barrier films; in particular, the invention relates to films and coatings with improved barrier characteristics produced by combining precursor deposition and atmospheric glow-discharge plasma treatment with various curing steps and pre- and/or post-deposition treatments tailored to optimize end results specific to each particular applications.
2. Description of the Related Art
As detailed in U.S. Ser. No. 10/228,358, glow-discharge plasma treatment has been an effective method of treating surfaces to increase their wettability and adhesion to various materials. Glow discharge provides a uniform and homogenous plasma that produces a consistent surface treatment with high-energy electrons that collide with, dissociate and ionize low-temperature neutrals, thereby creating highly reactive free radicals and ions. These reactive species enable many chemical processes to occur with otherwise unreactive low-temperature feed stock and substrates. Based on these properties, low-density glow-discharge plasmas are usually utilized for low material-throughput processes involving surface modification.
These plasmas are typically formed by partially ionizing a gas at a pressure well below atmosphere. For the most part, these plasmas are weakly ionized, with an ionization fraction of 10−5 to 10−1, established with AC or DC power in systems with varied geometries. These systems require vacuum chambers and pumps to maintain a very low pressure, which increases operating and maintenance costs. Accordingly, there has been an extensive effort in recent years to develop plasma systems capable of operating at atmospheric pressure for surface treatment of polymer films, foils, and paper.
It is known that atmospheric plasma can be generated at relatively low temperatures with a proper power source, the insertion of a dielectric layer between the electrodes, and the use of an appropriate gas mixture as the plasma medium. For surface treatment of polymer films, fabrics, paper, etc., atmospheric plasma can be established between two electrodes using an inert gas such as helium under particular operating conditions. Usually one electrode is attached to a high voltage power supply and a rotating drum is grounded and acts as the other electrode. One electrode is coated with a ceramic layer and the plasma gas is injected between electrodes. Examples of such glow-discharge plasma systems operating at atmospheric pressure are described in U.S. Pat. Nos. 5,387,842, 5,403,453, 5,414,324, 5,456,972, 5,558,843, 5,669,583, 5,714,308, 5,767,469, and 5,789,145.
U.S. Pat. No. 6,118,218, incorporated herein by reference, disclosed a plasma treatment system capable of producing a steady glow discharge at atmospheric pressure with a variety of gas mixtures operating at frequencies as low as 60 Hz. The invention consists of incorporating a porous metallic layer in one of the electrodes of a conventional plasma treatment system. A plasma gas is injected into the electrode at substantially atmospheric pressure and allowed to diffuse through the porous layer, thereby forming a uniform glow-discharge plasma. As in prior-art devices, the material to be treated is exposed to the plasma created between this electrode and a second electrode covered by a dielectric layer. Because of the micron size of the pores of the porous metal, each pore also produces a hollow cathode effect that facilitates the ionization of the plasma gas. As a result, a steady-state glow-discharge plasma is produced at atmospheric pressure and at power frequencies as low as 60 Hz. In order for the electrode holes to operate effectively for producing an optimal glow discharge, their size must approach the mean free path of the plasma-gas at the system's operating pressure.
U.S. Pat. No. 6,441,553, herein incorporated by reference, discloses a further improvement in the art as a result of the discovery that the porous metallic layer of U.S. Pat. No. 6,118,218 may be used in conjunction with novel electrode arrangements to overcome the substrate-thickness limitations imposed by conventional plasma-treatment apparatus. In an exemplary embodiment, the invention consists of two metallic electrodes embedded side by side in a dielectric medium having an outer layer defining an exposed treatment space. One of the electrodes is made of a porous metal and serves as a conduit for introducing the plasma gas into the treatment space at substantially atmospheric pressure. The two electrodes are energized in conventional manner, using one of the electrodes as a ground, to create an electric field between them and produce a uniform glow-discharge plasma in the treatment space. Thus, the material to be treated can be exposed to the plasma so created without substantial limitation as to thickness, geometry and composition. By eliminating the need to maintain an electric field across the substrate being treated, the electrode assembly of the invention makes it possible to treat thick substrates and substrates of metallic composition that could not be treated with prior-art equipment. In addition, a powdery substrate can be treated by adding a shaker to a belt used to convey the substrate through the plasma field.
According to another advance in the use of atmospheric plasma disclosed in U.S. Pat. No. 6,441,553, herein incorporated by reference, vapor deposition is carried out in combination with plasma treatment by vaporizing a substance of interest, mixing it with the plasma gas, and diffusing the mixture through the porous electrode. A heater is provided to maintain, if necessary, the temperature of the electrode above the condensation temperature of the substance in order to prevent deposition during diffusion. Thus, plasma treatment and vapor deposition are carried out on a target substrate at the same time at atmospheric pressure.
The invention of U.S. Pat. No. 6,441,553 lies in the combination of vapor deposition and plasma treatment at atmospheric pressure using certain classes of evaporable liquid and solid materials to produce films and coatings with specifically improved barrier properties. Inasmuch as similar coatings have been produced using vapor deposition and plasma treatment under vacuum, many useful gases (i.e., vapors at ambient conditions) and vaporizable constituents are known from the prior art that can also be used advantageously in the atmospheric-pressure process of the invention (such materials are typically referred to as “precursors” in the art).
Copending U.S. Ser. No. 10/228,358, herein also incorporated by reference, provides a further development in the art of using atmospheric-plasma treatment to improve conventional deposition and surface treatment processes. A plasma gas at atmospheric pressure is used with various vapor precursors, such as silicon-based materials, fluorine-based materials, chlorine-based materials, and organo-metallic complex materials, to enable the manufacture of coated substrates with improved properties with regard to moisture-barrier, oxygen-barrier, hardness, scratch- and abrasion-resistance, chemical-resistance, low-friction, hydrophobic and/or oleophobic, hydrophilic, biocide and/or antibacterial, and electrostatic-dissipative/conductive characteristics.
The present invention is the result of further developments in the art. It discloses various atmospheric techniques wherein plasma treatment is combined with precursor deposition and other process steps common in the art, such as curing with ultraviolet, visible, or infrared light, electron-beam radiation, and pre- and/or post-deposition plasma treatment of the product.